1. Field of the Invention
This invention relates to a band-pass filter. More specifically, it relates to a band-pass filter with a plurality of LC-type resonators and dielectric substrates. Such a filter may be used in, for example, a portable telephone or mobile telephone terminal. And this invention also relates to a duplexer using the LC-type dielectric filter.
2. Description of the Related Art
A dielectric filter may be used as a band-pass filter in mobile or portable telephone subscriber terminals. A typical conventional dielectric filter has a substantially rectangular dielectric block. A plurality of resonators extend from the top surface to the bottom surface of the dielectric block. Such a dielectric filter is disclosed in U.S. Pat. No. 4,431,997, "Ceramic Band Pass Filter" or Japanese Koukoku publication No. 62-57122, "Dielectric Filter".
This type of dielectric filter requires forming a plurality of resonant holes, and metalizing the inner surfaces of the resonant holes. Another type of band-pass filter is also known, and has a dielectric substrate, strip line resonators disposed on the substrate, and coupling circuits connected between the resonators. This type of filter is disclosed in Japanese Laid-Open Patent No. 62-164301, or No. 3-173201. To manufacture this type of filter, a process similar to that used to make printed-circuit boards can be employed. Accordingly, compared to the "rectangular type" dielectric filter, it is easier to manufacture this type of filter. Further, this type of filter can be manufactured to have a small size. However, because a plurality of resonators are on the same dielectric substrate, floating or stray capacitance occurs between the resonators, and thus, the stability of the coupling circuit is impaired.